1. Field of the Invention
The present invention relates to a thermal printer and a thermal head control method. More particularly, the present invention relates to an improvement of a thermal printer and a thermal head control method, in which a thermal head can be so operated that coloring density of recording material is prevented from being influenced by irregular escapement of heat.
2. Description Related to the Conventional Art
There are thermal transfer printers and direct thermal printers known in the art. The thermal transfer printers are classified into a thermal die transfer type and a thermal wax transfer type. In either of them, an ink ribbon or ink film is overlapped on a recording medium. A thermal head is pressed against the ink film toward the recording medium and at the same time driven to apply heat to them. The ink of the ink film is transferred to the recording medium. In the direct thermal printer on the other hand, a thermosensitive recording material is heated by a thermal head, directly to develop color in the thermosensitive recording material, to form an image on it.
The thermal head of the thermal printer consists of plural heating elements. For the recording of one pixel, the heating elements are supplied with at least one drive pulse for bias heating (bias pulse) and plural drive pulses for image heating (image pulses) in a number associated with density for the pixel of a halftone image or a character image. The bias pulses are used for heating the recording material to a temperature at which development of color starts in the thermosensitive material or before transfer of ink of the ink film. In the direct thermal printer, a color thermosensitive recording sheet is mounted on the periphery of a platen drum. The thermal head is contacted on the recording sheet. The platen drum is driven to make three rotations. In each rotation one of the three colors is recorded on the recording sheet, so that the three colors are recorded in frame-sequential fashion. A full color image is obtained on the recording sheet. The color thermosensitive recording sheet is disclosed in U.S. Pat. No. 4,734,704 (corresponding to Japanese Patent Laid-open Publication No. 61-213169), and consists of a support and cyan, magenta and yellow thermosensitive coloring layers formed on the support in the order listed.
In the recording sheet, the cyan coloring layer, which is the closest to the reverse surface of the recording sheet, has the lowest heat sensitivity of the three coloring layers, and requires considerably high heat energy to be colored. In the recording, great heat before escapement is accumulated in the whole of the thermal head, so that the accumulated heat of the thermal head influences distribution of the temperature through the heating elements. Such influences are remarkable in the recording with the coloring layers closest to the reverse side of the sheet. To be precise, the thermal head has lower temperature in the end positions than in the middle position, as viewed with reference to the main scanning direction, because greater areas around the ends of the thermal head are subjected to ambient air. The density of the printed image is lowered to an unwanted extent in the end positions relative to the main scanning direction. The coloring layers in the recording sheet are different in the heat sensitivity. In the thermal head having been driven, heat remains before escapement. Upon the recording of the colors, the remaining heat in the thermal head is different between the colors. There occurs a problem in that the gray balance of the reproduced image is irregular in the end positions relative to the main scanning direction. The fidelity in reproduction in color is remarkably low.